1. Field of the Invention
The present invention generally relates to a method and an apparatus for indicating the radioactive decay products of radium in an environment and, more particularly, but not by way of limitation, to a method and an apparatus for determining parameters relating to radon and the decay products of radon in an environment, such as the working level and the radon concentration, for example.
2. Brief Description of the Prior Art
In general, radioisotopes decay spontaneously and the decaying of the radioisotopes is accompanied with a radiation emission. Uranium (.sup.238 U) undergoes a series of successive radioactive transformations and each uranium decay product in the radioactive decay series is derived from the disintegration of the preceding element in the radioactive series, uranium ultimately decaying to a final or end product which is not radioactive [lead (.sup.296 Pb)]. The fifth decay product of uranium is radium (.sup.226 Ra) and radium has a relatively long half-life or half-period of approximately 1620 years (the half-life of a radioactive element being the time required for half of the initially present atoms to decay). The first decay product of radium is radon (.sup.222 Rn) and radon is the only element in the uranium decay series which is a gas at ordinary temperatures.
Since rocks and soils are porous, some of the radon diffuses out of the rock and soil surfaces into the surrounding environment on air, and the resulting airborne radiation is potentially hazardous to health. Particular airborne radon decay products (RaA, RaB, RaC and RaC') are considered to present the greatest danger to human health since a relatively substantial percentage of these inhaled airborne radon decay products are retained in the lungs. This is a particularly important consideration where the airborne radon and the airborne radon decay products are encountered in an underground mine environment.
Various parameters have been developed to measure or indicate the radioactivity due to the radon and the radon decay products with respect to the radiation exposure to the human lung and some of these parameters have been utilized for the purpose of establishing and maintaining safe working environment criteria. One such parameter has been referred to in the art generally as the "working level" (WL) and one (1) working level unit has been defined as the quantity of radon decay products (principally RaA, RaB, RaC and RaC'), in any mixture of such radon decay products, in a liter of air which produces (1.3)(10).sup.5 MeV. (million electron volts) of alpha particle energy as a result of the complete decay of radon through the fourth decay product (RaC') of radon. The continuous exposure of a worker to an environment having a working level of one (1) for forth (40 hours per week over a one (1) month period of time creates a "dose" of one (1) working level month (WLM). The federal government and various state governments have enacted regulations establishing radiation activity standards and, in general, the maximum airborne radioactivity levels, expressed in terms of a working level month (WLM), have been substantially lowered over the past fifteen years, such as from maximum levels in order of 120 WLM per year to maximum levels in the order of 4 WLM per year, for example.
The radiation exposure due to external or penetrating radiation and inhalation has become an extremely important consideration with respect to the working environment of various personnel and particularly with respect to the personnel working in radioactive material underground mines. In an effort to protect such personnel from an excessive, harmful exposure to radiation, the quality and stability of mine ventilation techniques have been improved over the last few years. To maintain the desired quality and stability of the mine ventilation techniques, it generally is necessary to maintain and establish programs and procedures for monitoring the mine ventilation equipment and the environment or air in the mine passageways.
Various analytical procedures and various types of equipment have been developed for detecting the radiation activity of air environment in the mine passageways. However, various problems have been encountered in attempting to develop a useful, effective and efficient technique and apparatus for detecting and measuring the required environmental parameters for determining the radiation activity in a mine environment. For example, distortion has presented a problem in detecting and measuring the required parameters, gamma ray background radiation has presented a problem, the equipment costs have been relatively high and sophisticated computation generally was required to analyze the various detected parameters for the purpose of obtaining the desired radiation activity parameter, such as the working level or the like, for example. Further, most of the prior techniques utilized for monitoring radon decay product (radon daughter) activity in mine environments have been relatively slow (35 to 90 minutes, for example), and the time delay generally encountered between the sampling of the mine air environment at particular test sites or locations and the determination of the desired parameter has been approximately two (2) hours, for example. Unfortunately, the time delay generally encountered in utilizing current techniques for detecting radon decay product activity greatly affects the ability to efficiently and economically maintain and control the quality of the mine air environment or, in other words, if the radon decay product activity could be evaluated in a relatively short time at the test site, remedial steps could be immediately effectuated to correct the condition causing any detected radon decay product activity near or in excess of a maximum level for safe working conditions. In addition, the mine air environment could be immediately re-sampled after such corrective measures have been completed for the purpose of evaluating the effectiveness of such corrective measures, thereby substantially reducing the possibility of a mine shut-down by the discovery and remedy of radiation conditions before such conditions result in an unsafe exposure to the mine air environment.
Some of the various fundamental physical principles relating to the behavior of radon daughters (decay products) were discussed in an article entitled "Engineers' Guide to the Elementary Behavior of Radon Daughters" by Robley D. Evans, Health Physics, Pergamon Press, 1969, Vol. 17, pp. 229-252. One prior art technique was discussed in an article entitled "Modification of the Tsivoglou Method for Radon Daughters in Air" by Jess W. Thomas, Health Physics, Pergamon Press, 1970, Vol. 19 (Nov.), p. 691.